gettext: Add a binding for setlocale()

This adds a Gettext.setlocale() binding for the setlocale() function, which is
sometimes needed in programs that use Gettext.

Also adds a Gettext.LocaleCategory enum which contains all the LC_... values
from locale.h, for use as the first parameter of this function.

https://bugzilla.gnome.org/show_bug.cgi?id=753072